A VCO is an important functional block in many types of analog, digital, and mixed-signal circuits. VCOs are used in PLLs in a wide variety of applications, such as clock generation, tuning circuits, and frequency synthesis. There are many types of VCOs and PLLs.
In one type of PLL, a phase-frequency detector (“PFD”) compares a reference clock signal to a feedback clock signal generated by a VCO to determine whether a charge pump needs to increase or decrease the charge in a loop filter. The loop filter produces a control voltage (“VCTRL”) that controls operation of the VCO and the clock signal produced by the VCO, thus completing the loop to maintain lock between the reference clock and feedback clock signals.
The PLL operates off of a clock signal. If the clock signal is lost, the operation of the VCO typically drifts off the lock frequency. Several responses to clock loss are possible. In some applications, the PLL circuitry is disabled by control logic that senses the loss of the clock signal to the VCO. The charge pump driving the loop filter of the PLL system is typically disabled, and the control (output) voltage of the VCO can drift. In another approach, the PLL circuitry remains active and the charge pump typically drives the loop filter to a supply rail depending on whether a reference clock or a feedback clock is lost. In another technique, a clamp circuit is activated when the clock is lost, and the clamp circuit holds the PLL at an operating point between the supply rails.
In each of the above cases, the voltage of the loop filter can be significantly different from that when the PLL is locked. This increases the time it takes to re-lock the PLL when the clock signal becomes available again. Additionally, it is possible for the VCO control voltage to be higher than the required lock voltage, which can cause problems in the PLL application.
Therefore, techniques for operating VCO-based circuits when the VCO clock signal is lost that avoid the problems of the prior art are desirable.